The present invention relates to a process for preparing polyethylene naphthalate (hereafter as "PEN") based polymers. More particularly, the present invention relates to a novel and advanced process for preparing PEN based polymers by using a composite polymerization catalyst comprising a titanium compound, a lead compound and an antimony compound.
Of polymeric polyesters, PENs have superior properties compared to polyethylene terephthalate (hereinafter as "PET"), the PENs are most widely known and commercially important because of their higher degree of crystallization, higher softening point, and other various superior properties in terms of strength, chemical resistance, thermal resistance, weather resistance, electric insulation, etc. Such PENs are widely used for manufacturing high quality films, bottles and high strength fiber and other industrial materials.
There are two steps in preparing PEN in the industrial preparation method. The first step is performed by means of either a direct esterification or an ester-interchange reaction. In the ester-interchange reaction, 2,6-naphthalene dicarboxylic acid dimethylester (2,6-NDC) and ethylene glycol (EG) are reacted in the presence of catalysts such as zinc acetate [Zn(OAc).sub.2 ] or manganese acetate [Mn(OAc).sub.2 ] at reaction temperatures ranging from 180 to 260.degree. C. to remove methanol. In the direct esterification reaction, 2,6-naphthalene dicarboxylic acid (2,6-NDCA) and ethylene glycol (EG) are heated to reaction temperatures ranging from 200 to 280.degree. C. under atmosphere or pressure to remove water. The product of the first step is bis(beta-hydroxyethyl)naphthalate and/or its low prepolymer (hereinafter as "esterified compounds") and thereafter in the second step, the synthesized, esterified compounds are polycondensed in the presence of polymerization catalysts such as antimony trioxide (Sb.sub.2 O.sub.3) at a higher reaction temperatures ranging from 280 to 300.degree. C. under reduced pressure (generally less than 1.0 torr) in order to prepare the high polymers.
Until recently, ester-interchange reaction has been mainly adopted for industrial preparation of PEN because of the high price of the starting materials of the direct esterification method. However, direct esterification is expected to be adopted widely when 2,6-NDCA is industrially mass-produced at an economical price.
Similar to the PET preparation, reaction catalysts are generally used to accelerate and smoothly advance a reaction in preparing PEN. These catalysts include a variety of metal compounds such as antimony, titanium, germanium, tin, zinc, manganese, lead and the like. It is well-known to those skilled in the art that the color and thermal stability of the resulting PEN and the reaction rate are considerably varied depending on the catalysts used. The reactions for preparing PENs are carried out at high temperatures for an extensive period in the presence of catalysts containing metals.
Accordingly, the reactions for preparing PENs of a high degree of polymerization in a short time are accompanied by several undesirable side reactions that result in coloring the polymer product yellow and increasing the amount of diethylene glycol and the concentration of terminal carboxylic groups above their optimum levels. Consequently, the physical properties of the prepared PENs, for example, the melting point, strength and the like are deteriorated. Therefore, it is important to prepare polymers that can exhibit good color and superior physical properties even at a high reaction rate.
At the present time, antimony compounds, especially antimony trioxide, is mainly used as an industrial polycondensation catalyst, since it is inexpensive and exhibits good catalytic activity and good thermal stability. However, antimony trioxide is basically not soluble in ethylene glycol or other reaction mixtures and tends to precipitate during the reaction, thereby causing the resulting color of the PEN is gray or yellow-green or the transparency thereof is decreased. These are more distinct if the amount of the catalyst used and the reaction temperature are increased to improve the productivity.
In order to provide catalysts for solving the above-mentioned problems, there have been several methods proposed to reduce the esterification reaction time and the polycondensation reaction time and to produce polyesters exhibiting good color and superior physical properties. However, many of the methods could not solve the above-mentioned problems: a method of dissolving antimony trioxide, a compound of cobalt and a compound of phosphorous in ethylene glycol (Japanese Laid-Open Patent No. Sho 53-51295) and a method in which compound of antimony is used with an organic acid (Japanese Laid-Open Patent No. Sho 60-166320) were tried. However, these methods cannot substantially reduce both the esterification reaction time and the polycondensation reaction time. They also generate several problems in physical properties of the prepared polyesters, in that the color of the prepared polymer is light yellow or the content of the diethylene glycol or terminal carboxylic groups are increased. Also, as a method for improving the color and physical properties of the prepared polymer, there have been known, for example, a method in which compounds of cobalt and alkali metal are used with a compound of antimony (Japanese Laid-Open Patent No. Sho 58-117216), a method in which a compound of antimony is used with a compound of tin (Japanese Laid-Open Patent No. Sho 49-31317), and a method in which antimony, tin, cobalt and alkali are used with a compound of phosphorous (Japanese Laid-Open Patent No. Sho 62-265324). However, these methods can not improve the color, transparency and physical properties of the prepared polymer at the same time and can not provide any important advantage in terms of the reduction of the reaction time. On the other hand, according to the previous inventions by the present inventors, these problems of preparing polyesters, especially PET have been overcome.
In these inventions, PETs are prepared by using a compound of titanium and a compound of antimony (U.S. Pat. No. 5,286,836) and by using additional tin compound along with a compound of titanium and a compound of antimony to form a composite catalyst system (U.S. Pat. No. 5,714,570). However, a marked difference between the preparation of PEN and PET exists. For instance, the reactant, 2,6-NDCA has a lower solubility in EG, higher molecular weight and smaller crystal sizes than TPA(terephthalic acid). Therefore, it is not possible to feed the poorly made slurry into the reactor if 2,6-NDCA and EG is used in similar molar ratio of PET preparation (generally, EG/TPA=1.1.about.2.5). Moreover, more care is needed in preparing PEN since the naphthalene ring of 2,6-NDCA or 2,6-NDC is more prompt to colorize by impurities than the benzene ring of PET, and PEN has a higher melt viscosity when melted and has a higher polymerization temperature than PET. Other methods to overcome these problems have been introduced (WO 90-14375 and WO 97-17391), however, without much success in shortening the reaction time or in improving the productivity.